How to use a USB flash drive in Python
How to access an attached USB flash drive in Python
This article is about using the Attached USB Flash drive in Python, methods of reading and writing to it, and the current limitations of such.
See this Wiki page for information on FLASH filesystem statistics: Estimating free flash file space.
Hardware requirements
- A Digi USB/Python Enabled device. Typically a ConnectPort X4 or ConnectPort X8 device.
- A Compatible USB flash drive. Many flash drives are compatible, however any which include hardware (or Windows-specific) encryption or compression will not work. Also, the flash drive must be 4GB or smaller due to FAT filesystem limitations. To check, see 'Detecting your USB flash drive' section below.
- Command line interface access to the Digi device.
Digi Connect transport comments
In the examples below, the USB Flash drive is indicated by 'A/'. The Transport line (such as WR21 or WR44) use 'u:' instead. So instead of "A/tmp.txt", you would use "u:tmp.txt" - plus are restricted to the smaller 8.3 DOS-like names. Instead of duplicating the example, just use your imagination to see 'u:' instead of 'A\' in the examples below.
Detecting your USB flash drive
To detect your USB flash drive, attach the USB drive to an external USB port of the Digi device. Connect to the command line interface of the device, and type the command flashdrv. If a USB drive is detected, it will display volume information such as: The name of the USB flash drive, total space, used space, available space, and which volume it is mounted as. Make note of which volume the device is mounted as, typically A. Drive volume letters are assigned starting with A and progressing sequentially through the alphabet. It is not recommended that more than one drive be used, without an additional mechanism to identify the volume label, because the enumeration process may assign different letters on boot in this case.
Here is an example of the output:
#> flashdrv Volume Bytes Used Available Bad Use% Mounted on DISK_IMG 127.63 MB 124.09 MB 3.54 MB 0.00 MB 97.4 A/
Writing and reading to your USB flash drive
To write and read from the USB flash drive, the volume the drive is mounted at is needed. For the example below, we will be assuming your drive is mounted as A.
fh = open("A/tmp.file", 'w')
fh.write("Foobar")
fh.close()
fh = open("A/tmp.file", 'r')
print (fh.read())
The above code opens a file on the volume mounted at A, writes to it, closes it, then reopens it and reads back the data within. If the file does not exist, the file will be created. The write method is destructive and will overwrite existing data. Use the append method below if needed.
fh = open("A/tmp.file", 'a')
fh.write("Foobar 2")
fh.close()
fh = open("A/tmp.file", 'r')
print (fh.read())
The above code opens a file on the volume mounted at A, appends it, closes it, then reopens it and reads back the data within. Data by default appended sequentially. If a carriage return is desired use "\n".
Limitations of the USB flash drive
Similar to built in flash of the Digi product, the supported Python os commands are listed in the Digi Python Programming guide. In addition to that, there is no way currently in the web user interface and command line interface to list the contents either.
To test whether or not a file exists, you must attempt to open it. The exception that is generated will show you whether or not the file exists. If the file does not exist, an "IOError: [Errno 13] Permission Denied" exception will be generated.
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